Tunnel boring machines construct underground tunnels having a diameter ranging between a fraction of a meter up to several meters. The tunnel boring machine and its operating crew can perform several functions simultaneously to construct the tunnel, including boring, tailings material removal, lining, and installation of utilities into the tunnel such as fresh air conduits, power and water supply, etc.
The boring function of the typical tunnel boring machine is performed by a large rotating head provided at the forward end of the machine. The head rotates around an axis generally coaxial with the tunnel geometry. The rotating head gradually removes the material in the path of the machine at the face of the advancing tunnel. As the face of the tunnel is excavated and the debris removed, the tunnel length increases and the tunnel boring machine continuously advances to maintain the engagement of the head with the face. Cutters mounted to the rotating head perform the task of excavating the material from the face so that it can be collected and removed by the head and a conveyor system into aft portions of the machine for storage and/or transport out of the tunnel. The head advances and the cutters are pushed against the face typically under power from a system of hydraulic cylinders. Hydraulic cylinders are also deployed along with means which push against the sides of the tunnel in order to react the force of the cutters against the tunnel face.
Tunnel boring machine heads have utilized a variety of cutter styles. Fixed pick style cutters may be used in soft materials. In hard materials like hard rock, rotary cutters have typically been used. A number of rotary cutters are mounted in a pre-established pattern onto the head so that as the head rotates, a cutter is able to contact each portion of the face, engaging and removing material at a roughly equal rate across the area of the face. Rotary cutters employ a cutting ring mounted via a bearing onto a shaft. The shaft is in turn secured on the cutting head. As the head rotates, the cutting ring rotates on the shaft. The cutting ring is relatively sharp. As the ring pushes against the tunnel face with great compressive force, the rock adjacent the cutter ring is crushed and sheared and falls off of the face and is collected and removed as debris.
The service life of these rotary cutters can be a significant limitation in the operating efficiency of the tunnel boring machine. The cutters are pushed against the face with very significant forces including high shock loads and work in an abrasive, high wear environment. Thus, the cutter rings can be worn at a rapid rate. The cutter rings may be replaced after they are worn. But to change the cutter rings, the machine must be stopped for several hours while the cutters are removed and new cutter rings are installed. This time intensive re-ringing activity reduces the overall efficiency or rate of excavation of the machine.
Also, the bearing system between the cutter ring and the shaft can fail and require premature replacement of the entire cutter before the cutter rings have been worn. When the bearing system fails, the cutter ring stops turning. When the cutter ring stops turning, the portion of the cutter ring in contact with the face slides, the sliding contact wearing the cutter ring rapidly into a flat, wide spot which no longer has adequate compressive forces to crush the hard rock face.
One example of a typical rotary cutter design is seen in U.S. Pat. No. 4,793,427, (“the '427 patent”) issued in 1988 to Boart International Limited. Other examples of cutter designs are found in U.S. Pat. No. 6,131,676 (“the '676 patent”) issued in 2000 to Excavation Engineering Associates, Inc. Many different types and styles of rotary cutters, in addition to those in the '427 patent and the '676 patent, have been proposed and tested. But today the cutter remains one of the most important wear items on a tunnel boring machine and similar equipment, and constitutes an important limiting factor on the machine's excavation speed and efficiency.